Change shebang to use /usr/bin/env python3

Depending on the user's setup, python3 may not be in
/usr/bin, especially if they use something like
Homebrew, which uses /usr/local/bin. Using `env`
works around this, and is more likely to find a working
python3 installation.
Use /usr/bin/env python3 in cherrymusicserver package (#450)